Cleanup js -> string

Converting a JS value to a string is a bit messy right now. There's duplication
between string helpers in js.Local, and what js.String and js.Value provide.

Now, all stringifying functions are in js.String, with some helpers in js.Value.

Also tried to streamline the APIs around most common-cases (e.g. js.String ->
[]u8 using call_arena). js.String now also implements format, so it can be
used as-is in some cases.

src/main_wpt.zig

@@ -140,7 +140,7 @@ fn run(
         return err;
     };
 
-    return value.toString(.{ .allocator = arena });
+    return value.toStringSliceWithAlloc(arena);
 }
 
 const Writer = struct {